NullPointerException when Inserting Data using ManyToMany in SpringBoot Application

Question

I have tried all I could but I keep getting NullPointerException Error.

I have two Spring boot entities that have been mapped together using ManyToMany mapping. These Entities are fine as all the tables are created.

However, when I try to insert data, I get a NullPointerException.

Can someone please point me to what I am doing wrong?

Below is the code that I'm using to insert the data:

import com.ait.aiadmin.model.Cluster;
import com.ait.aiadmin.model.Subscriber;
import com.ait.aiadmin.repository.ClusterRepository;
import com.ait.aiadmin.repository.SubscriberRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.stereotype.Component;

import javax.transaction.Transactional;

@SpringBootApplication
public class AiAdminApplication {
    public static void main(String[] args) {
        SpringApplication.run(AiAdminApplication.class, args);
        /*ApplicationContext ctx = SpringApplication.run(AiAdminApplication.class, args);

        String [] beanNames = ctx.getBeanDefinitionNames();
        Arrays.sort(beanNames);
        for (String name: beanNames){
            System.out.println(name);
        }*/
    }

    @Component
    public class DatabaseLoader implements CommandLineRunner {
        private final SubscriberRepository subscriberRepository;
        private final ClusterRepository clusterRepository;

        @Autowired
        public DatabaseLoader(SubscriberRepository subscriberRepository, ClusterRepository clusterRepository) {

            this.subscriberRepository = subscriberRepository;
            this.clusterRepository = clusterRepository;
        } 

        @Override
        @Transactional
        public void run (String...strings) throws Exception {

            Subscriber subscriber1 = new Subscriber("Olalekan Samuel", "Ogunleye", "olalekan@gmail.com");
            Cluster cluster1 = new Cluster("Aws-eu-west-1", "Ireland", "123.98.45", "Olalekan Samuel");
            subscriber1.clusters.add(cluster1);
            this.subscriberRepository.save(subscriber1);
            this.clusterRepository.save(cluster1);
    }
}

And below is the error that I am getting

java.lang.IllegalStateException: Failed to execute CommandLineRunner at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:735) [spring-boot-1.5.8.RELEASE.jar:1.5.8.RELEASE] at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:716) [spring-boot-1.5.8.RELEASE.jar:1.5.8.RELEASE] at org.springframework.boot.SpringApplication.afterRefresh(SpringApplication.java:703) [spring-boot-1.5.8.RELEASE.jar:1.5.8.RELEASE] at org.springframework.boot.SpringApplication.run(SpringApplication.java:304) [spring-boot-1.5.8.RELEASE.jar:1.5.8.RELEASE] at org.springframework.boot.SpringApplication.run(SpringApplication.java:1118) [spring-boot-1.5.8.RELEASE.jar:1.5.8.RELEASE] at org.springframework.boot.SpringApplication.run(SpringApplication.java:1107) [spring-boot-1.5.8.RELEASE.jar:1.5.8.RELEASE] at com.ait.aiadmin.AiAdminApplication.main(AiAdminApplication.java:19) [classes/:na] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_152] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_152] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_152] at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_152] at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49) [spring-boot-devtools-1.5.8.RELEASE.jar:1.5.8.RELEASE] Caused by: java.lang.NullPointerException: null at com.ait.aiadmin.AiAdminApplication$DatabaseLoader.run(AiAdminApplication.java:73) ~[classes/:na] at com.ait.aiadmin.AiAdminApplication$DatabaseLoader$$FastClassBySpringCGLIB$$d20028d3.invoke() ~[classes/:na] at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:204) ~[spring-core-4.3.12.RELEASE.jar:4.3.12.RELEASE] at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:738) ~[spring-aop-4.3.12.RELEASE.jar:4.3.12.RELEASE] at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157) ~[spring-aop-4.3.12.RELEASE.jar:4.3.12.RELEASE] at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:99) ~[spring-tx-4.3.12.RELEASE.jar:4.3.12.RELEASE] at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:282) ~[spring-tx-4.3.12.RELEASE.jar:4.3.12.RELEASE] at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96) ~[spring-tx-4.3.12.RELEASE.jar:4.3.12.RELEASE] at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.3.12.RELEASE.jar:4.3.12.RELEASE] at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:673) ~[spring-aop-4.3.12.RELEASE.jar:4.3.12.RELEASE] at com.ait.aiadmin.AiAdminApplication$DatabaseLoader$$EnhancerBySpringCGLIB$$6e72d57b.run() ~[classes/:na] at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:732) [spring-boot-1.5.8.RELEASE.jar:1.5.8.RELEASE] ... 11 common frames omitted

Answer 1

I don't know your code for the Subscriber, but I would guess the NPE occurs, because subscriber1.clusters is not initialized. Make sure that all Collections in your entities are created correctly.

In my applications I often use the getter for a collection to initialize it, if it does not exist, e.g.:

public List<Something> getSomethings() {
    if (somethings == null) {
        somethings = new ArrayList<>();
    }
    return somethings;
}